Project Summary Although potent antithrombotic drugs are available, all inadvertently target vital hemostatic mechanisms, resulting in dose-limiting hemorrhagic toxicity that restricts their use. Due to a lack of safe thromboprophylaxis, thrombotic/thromboembolic blood vessel occlusions and vascular device failures remain among the leading causes of death and severe chronic disability in the U.S. Consequently, there is a significant and urgent unmet medical need for safe antithrombotic drugs. The safety problem with current antithrombotics is particularly complicated in end stage renal disease (ESRD) patients on chronic hemodialysis, who are prone to both bleeding and thromboembolic complications. Moreover, some ESRD patients develop acute heparin induced thrombocytopenia (HIT), another potentially life threatening complication of heparin use in a small but significant fraction of ESRD patients, leaving them with few if any options for temporal anticoagulation during hemodialysis sessions. We will therefore continue clinical development of our first-in-class, FDA Fast Track designated antithrombotic enzyme, ProCase (E-WE thrombin, AB002), by beginning to explore its safety and antithrombotic activity during hemodialysis. The product candidate is a hemostatically safe antithrombotic protein C activator enzyme that has the potential to help this desperately ill patient population. E-WE thrombin has been designed to act in part by increasing the surface concentration of the anticoagulant, profibrinolytic, and cytoprotective enzyme, endogenous activated protein C (APC), at the site of developing blood clots via targeted cellular delivery. This unique mechanism of action allows E-WE thrombin to target cell-rich pathological blood clots (thrombi) without disabling vital hemostasis. In primates, doses as low as 1 g/kg are antithrombotic without significant systemic anticoagulation or measurable antihemostatic effects. This critical Phase I/II Fast-Track SBIR grant will allow us to continue product development by providing essential support for an exploratory phase 2a clinical study to evaluate the safety and antithrombotic effect of E-WE thrombin during hemodialysis in a small group of ESRD patients. The trial will be deemed successful and support further studies in this and other indications if E-WE thrombin is not associated with an increase in clinically significant drug-related adverse events versus placebo, while showing evidence of antithrombotic activity in the hemodialysis device. Successfully achieving our SBIR milestone will lead directly into the next product development stage: performing subsequent definitive trials in hemodialysis and other important blood clotting diseases (e.g. ischemic stroke, pulmonary embolism, and acute myocardial infarction) for the benefit of patients who desperately need safer antithrombotic and thrombolytic therapies.